SAMM 368, a myeloma which secretes IgG2b and IgA paraproteins simultaneously, shows cytoplasmic segregation of the gamma and alpha heavy chains, and rapid assembly of IgG2b monomers and IgA mutlimers, the latter bonded to J chains in a late stage of assembly. Several B-cell tumor have surface-IgD as well as surface IgM by immunofluorescence. Synthesis of IgD has been demonstrated in these tumors.